The continuing evolution in the medical field of the study and control of cardiac activity requires advances in the designs of various implantable devices utilized to control cardiac activity. Various types of implantable leads utilize electrodes, which are currently either implanted into an interior chamber of the heart or which are affixed to the exterior surface of the heart, or within or upon the pericardial sac which surrounds the heart. These devices, depending upon their intended implant location, have various specific requirements. For the devices designed to be affixed to the exterior surface of the heart, or implanted within or upon the pericardial sac, the environment places specific design constraints on the materials and construction of the devices.
For patch electrodes which are affixed to the external surface of the heart, or within or on the pericardial sac, the environment requires that the electrode be highly resistant to fractures caused by the flexing resulting from the continuous beating of the heart. In addition, the electrodes must exhibit high conductivity, high flexibility, and biological inertness. For various types of cardiac electrodes, the use of very fine titanium or platinum wire mesh, which provides very good electrical properties while being able to conform to the shape of the heart, has become the material of choice.
Additionally, when using electrodes which are placed directly on the heart, it is a requirement that the electrical conductors interconnecting the patch electrode and a signal processing and power generating assembly, be very flexible and resistant to fracture from repeated flexion stresses. As may be appreciated, the heart and the body both undergo continuous movement which cannot be inhibited by the conductors or the electrodes affixed to the surface of the heart. In addition, in recognition of the critical nature of the devices, any fracture or degradation in the electrical performance of the electrode and/or the conductors, is unacceptable.
The electrical characteristics of the design of the conductor element are also critical because of the requirements that the conductor transmit both low voltage sensing signals and high defibrillation currents. Thus, the tip-to-tip resistance and the current capacity properties are as important as the flexure and durability characteristics. Further, it is desirable that the conductors have some degree of redundancy in the event that a failure in a part of the conductor does occur, so that the defibrillator patch will not be rendered inoperative.
The various types of materials for the electrodes and conductors, as well as various specific design considerations, influence the method of affixing the conductors to the wire mesh of the electrode patch. The particular difficulty involves providing secure electrical and mechanical contact between the electrical conductors and the electrode mesh.